1. Field of the Invention
This invention relates to improving the separation of the catalyst phase from the gasiform phase in a catalytic cracking unit. More particularly, it relates to improvements in separating the catalyst phase from the gasiform phase as the suspension containing them discharges from a riser outlet. This invention especially relates to improving the separation of the gasiform product phase from the catalyst at the time the desired chemical reaction has been accomplished in the presence of the catalyst.
2. Description of the Prior Art
The field of catalytic cracking, particularly fluid catalyst operations, has undergone significant development improvements due primarily to advances in catalyst technology and product distribution obtained therefrom. With the advent of high activity catalysts and particularly crystalline zeolite cracking catalysts, new areas of operating technology have been encountered requiring even further refinements in processing techniques to take advantage of the high catalyst activity, selectivity and operating sensitivity.
Of particular concern in this field has been the development of methods and systems for separating gasiform products from fluidizable catalyst particles, particularly from a high activity crystalline zeolite cracking catalyst, under more efficient separating conditions so as to reduce the overcracking of conversion products and promote the recovery of desired products of a hydrocarbon conversion operation. However, present day cyclonic equipment often permits an undesired extended contact between the product phase and the catalyst phase of a portion of the suspension discharged. This extended contact has been found to contribute to a loss in desired product amounting up to about 10 percent. Thus, it has been determined that the gasoline yield loss due to overcracking can be in the range of 0.15 to about 1.5 volume percent based on fresh feed. Recent developments in this art have been concerned with the separation and recovery of entrained catalyst particles from gasiform products in a short contact time riser hydrocarbon conversion operation.
The hydrocarbon conversion catalyst employed is preferably a high activity crystalline zeolite catalyst of fluidizable particle size which is transferred in suspended or dispersed phase condition generally upwardly through one or more riser conversion zones providing a hydrocarbon residence time in each conversion zone in the range of 0.5 to about 10 seconds and more usually less than about 8 seconds. High temperature riser hydrocarbon conversions of at least 1000.degree. F. at 0.5 to 4 seconds hydrocarbon residence time in contact with the catalyst in the riser is desirable for some operations before initiating separation of vaporous hydrocarbon product materials from the catalyst. Rapid separation of catalyst from hydrocarbons discharged from a riser conversion zone is particularly desirable for restricting hydrocarbon conversion time. During the hydrocarbon conversion step, carbonaceous deposits accumulate on the catalyst particles and the particles entrain hydrocarbon vapors upon removal from the catalyst conversion step. The entrained hydrocarbons are subjected to further contact with the catalyst until removed from the catalyst by mechanical means and/or stripping gas in a separated catalyst stripping zone. Hydrocarbon conversion products separated from the catalyst and stripped materials are combined and passed to a product fractionation step. Stripped catalyst containing deactivating amounts of carbonaceous material hereinafter referred to as coke is then passed to a catalyst regeneration operation.
Various processes and mechanical means have been employed heretofore to effect separation of the catalyst phase from the hydrocarbon phase at the termination of the riser cracking zone.
U.S. Pat. No. 4,043,899 of Anderson, et al. disclosed a method for rapid separation of a product suspension comprising fluidized catalyst particles and the vaporous hydrocarbon product phase by discharging the entire suspension directly from the riser conversion zone into a cyclonic separation zone which provides cyclonic stripping of the catalyst after it is separated from the hydrocarbon vapors. In the method of Anderson, et al. the cyclone separator is modified to include an additional downwardly extending section comprising a lower cyclone stage. In this arrangement, catalyst separated from the gasiform material in the upper stage slides down a downwardly sloping baffle to the lower cyclone where stripping steam is introduced to further separate entrained hydrocarbon product from the catalyst recovered from the upper cyclone. The steam and the stripped hydrocarbons are passed from the lower cyclone through a concentric pipe where they are combined with the hydrocarbon vapors separated in the upper cyclone. The separated and stripped catalyst is collected and passes from the cyclone separator by conventional means through a dipleg. This process requires that the entire volume of catalyst, gasiform phase and stripping steam pass through the cyclone separator which means that this equipment must be designed to efficiently handle not only a large vapor volume but also a large quantity of solid particles.
U.S. Pat. No. 4,070,159 of Myers et al. provides a separation means whereby the bulk of the solids is discharged directly into the settling chamber without passing through a cyclone separator. In this apparatus, the discharge end of the riser conversion zone is in open communication with the disengaging chamber such that the catalyst discharges from the riser in a vertical direction into the disengaging chamber which is otherwise essentially closed to the flow of gases. The cyclone separation system is in open communication with the riser conversion zone by means of a port located upstream from but near the discharge end of the riser conversion zone. A deflector cone mounted directly above the terminus of the riser causes the catalyst to be directed in a downward path so as to prevent the catalyst from abrading the upper end of the disengaging vessel. The cyclone separator is of the usual configuration employed in a catalytic cracking unit to separate entrained catalyst particles from the cracking hydrocarbon products so that the catalyst passes through the dipleg of the cyclone to the body of the catalyst in the lower section of the disengaging vessel and the vaporous phase is directed from this vessel to a conventional fractionation unit. There is essentially no net flow of gases with the disengaging vessel beyond that resulting from a moderate amount of steam introduced to strip the catalyst residing in the bottom of the disengaging vessel.
U.S. Pat. No. 4,219,407 of Haddad et al. discloses the separation of the catalyst from the gasiform cracked products in a fashion which permits effective steam stripping of the catalyst. The suspension of catalyst and gasiform material is discharged from the riser conversion zone outwardly through radial extending passageways which terminate in a downward direction. Catalyst stripping zones are located beneath the terminus of each of the radial extending passageways. Each stripper consists of a vertical chamber open at the top and the bottom with downwardly sloping baffles located within the chamber so as to cause the catalyst to flow in a discontinuous manner countercurrent to upwardly flowing stripping steam introduced at the lower end of the stripping chamber. The radial extending arms are provided with a curved inner surface and confining sidewalls to impart a cyclonic concentration of catalyst particles promoting a forced separation from the hydrocarbon vapors discharged as a suspension from the riser. This separation of the catalyst from the vapors is basically achieved through rapid changes in direction of the several phases. Thus the cyclonic collection and concentration of catalyst particles is used to reverse the flow of separated catalyst such that it is concentrated as a downwardly confined stream which discharges generally downwardly and into the open upper end of the catalyst stripping chamber. A vapor disengaging space is provided between the discharge end of the radial extending arms and the top of the catalyst strippers to promote the rapid removal of separated vapors from out of contact with discharged catalyst. The separated vapors pass upwardly through the disengaging vessel to the open inlet of a cyclone separator which removes entrained catalyst from the gasiform material for return through a dipleg to the body of steam stripped catalyst while the separated vaporous material passes to a fractionation unit. The hydrocarbon product, as it passes within the disengaging vessel from the discharge of the radial extending arms to the cyclone separator, travels in a random fashion and is exposed to catalytic reaction temperatures which cause undesirable side reactions and thermal cracking before these vapors enter a quench zone in the main fractionator of the fluid cracking unit.
It is an object of this invention to provide a process and an efficient means for separating cracking catalyst from gasiform material.
It is another object of this invention to provide a process and an apparatus which will separate cracking catalyst and gasiform material in a manner which is an improvement over the presently available processes and apparatus, particularly the process disclosed in U.S. Pat. No. 4,219,407.
It is a further object of this invention to provide a process and an apparatus for separating cracking catalyst and gasiform material whereby the length of time the gasiform material is subjected to high temperature after separation from the bulk of the catalyst is minimized so as to reduce overcracking and thermal cracking of the cracked products.